This invention relates to standing wave systems and in particular to the manipulation of particles suspended in a fluid medium in an ultrasonic standing wave system.
It has been proposed to employ ultrasonic standing waves for the control of the position and movement of particles in a fluid medium, as is described for example in EPs 147032 and 167406. The effectiveness of such systems is of course dependent on the degree of resonance between intersecting progressive waves which create the standing wave. To obtain full resonance from a pair of opposed sources, it is necessary for the distance between the sources to be set precisely, the determinant being the acoustic path length between the sources in terms of the wavelength of their radiation.
The fully resonant state is achieved when the path length between two opposed sources, or between a source and reflector, is equal to an integral number of half-wavelengths. As the acoustic distance changes, in either sense, the resonant effect is weakened and is finally completely lost when it becomes equal to an odd integral number of quarter-wavelengths. The rate of weakening from full resonance as the deviation from fully resonant conditions increases is the greater if the damping coefficient of the system is low. Nevertheless, while damping in a water-filled acoustic cavity operating at high frequency may be fairly large, quite minor deviations from the path length required for the fully resonant condition will still produce a sharp fall in the resonant effect.
If the path of the progressive waves of the system is long, e.g. if it occupies hundreds of half-wavelengths, which can be required particularly in high frequency, e.g. MHz, systems, the percentage change of acoustic distance (i.e. the distance measured in wavelengths) to move from fully resonant to completely non-resonant conditions can be extremely small. Moreover, as the acoustic path length increases the relative change of length that will transform conditions from the fully resonant to the completely non-resonant becomes smaller. It is therefore increasingly probable that variations of physical conditions or inhomogeneities, e.g. temperature fluctuations, will create acoustic path length changes of the same or a larger order completely randomly.
Some idea of the effects on small variations of conditions can be gathered from the example of a standing wave at a frequency of 2 MHz and having a path length of 200 mm in water at 21.degree. C. and atmospheric pressure. At these conditions the sonic velocity in water is 1486.6 m/s, which gives an acoustic path length of 538.1 quarter-wavelengths, corresponding to an almost fully resonant condition. If the water temperature falls to 20.degree. C., the sonic velocity becomes 1484 m/s to give a path length of 539.1 quarter-wavelengths, resulting in an almost complete loss of resonance.
The performance of resonant systems can therefore be adversely and uncontrollably affected by such changes. Correction of the fault by a closed-loop control of the means generating the acoustic energy is not a practical proposition because of the transient and random nature of many of the influences at work. For these systems to operate reliably and consistently in conditions in which external influences may not be easily controlled, there is a need for a method of limiting adverse effects from the causes described.